US2151556A - Synchronous motor control system - Google Patents

Description

March 21, 1939.

A.. H. LAUDER 2,151,556

SYNCHRONOUS MOTOR'CONTROL SYSTEM Filed Jan. 29, 1937 Inventor: Arthur H. Lauder: b9 WW His Attorney.

Patented Mar. 21, .1939

UNITED STATES SYNCHRONOUS MOTOR common srs'rmu Arthur H. Lauder, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application January 29, 1937, Serial No. 122,935

5 Claims.

My invention relates to control, systems for synchronous motors, and particularly to such a system for controlling the excitation of a synchronous motor in response to the motor reaching a predetermined speed near synchronous speed during the starting operation, and its object is to provide an improved field excitation control system of this type.

Synchronous motors are sometimes used in such a manner that they have their maximum load applied thereto at the instant of starting and for a short time thereafter. For example, in a ball mill the material settles down in. such a manner when the mill is stopped that the load on the driving motor is ten to twenty per cent greater at the instant of starting than it is after the mill has been in operation fifteen or twenty seconds. When such a mill is driven by a synchronous motor which has a small margin of pull-in torque, it is necessary, in order to synchro-nize the motor, that it be allowed to attain its maximum possible speed before field excita-- tion is applied. Also, in such a case, the field excitation should be applied at the-most favorable point during the slip cycle. It is the usual practice to adjust the speed-responsive means which controls the application of excitation to a synchronous motor field winding so that the excitation is' applied at substantially the maxiiii) mum speed it can reach as an induction motor. The speed-responsive means cannot be safely set to operate at the maximum speed which the motor will attain as an inductionmotor because any slight increase in load or a reduction in the voltage applied tothe motor armature winding will prevent the motor from ever attaining this maximum speed, in which case the field excitation would never be applied.

In order to take advantage of all of the margin in pull-in torque possessed by the motor and to give the load an opportunity to reduce as much as possible before the motor is synchronized, I provide, in accordance with my invention, an arrangement for recalibrating the speed-responsive means after the motor has been in operation for a predetermined time so that, if for any reason the motor fails to reach within a predetermined time the predetermined speed at which the speed-responsive means is normally set to respond, the speed-responsive means is recalibrated to operate at a lower predetennined speed.

My invention will be better understood from the following description, when taken. in connection with the accompanying drawing, the single figure of which diagrammatically illustrates a synchronous motor starting system embodying my invention, and its scope will he pointed out in the appended claims.

Referring to the accompanying drawing, l represents a synchronous motor having an armature winding which is arranged to be connected to a suitable alternating current supply circuit 2 by means of a switch 3 which may be of any suitable type, examples of which are well-known in the art. While I have shown a manually controlled switch, it is obvious that any suitable automatic means maybe employed for controlling the opening and closing thereof. The motor I is also provided with a field winding 4, which is arranged to be connected to a suitable source of excitation 5, by means of a field switch 6 that is arranged to be closed when the motor 1 speed increases above a predetermined value.

The field switch 5, when open, is arranged to conmeet a suitable discharge resistor 5 across the terminals of the field winding i.

In order to close the field switch 8 automatically when the motor speed reaches a predetermined value during the starting operation of the motor, the field switch 6 is provided with a closing coil 8, the circuit of which is controlled by means of a relay 9, the winding of which is connected in series relation with a. half-wave rectifier Ill and the field winding 4 when the field switch 5 is open. Preferably the winding of the relay 9 and the rectifier iii are connected in, series across a portion or all of the resistor I so that the rectifier Ill only rectifies a portion of. the total induced current flowing in the field winding 4. The relay 9 may be of any suitable type, examples of which are well-known. in the art, which closes its front contacts substantially instantaneously when the relay winding is energized by a. predetermined current and which opens its front contacts a. predetermined time after the relay winding is deenergized.

In order to vary the calibration .of the speedresponsive relay 9 after the motor has been in operation for a predetermined time, I provide a suitable impedance means, such as a resistor H, in series with the rectifier Ill and the winding of the relay 9, and have this resistor H normally short-circuited by the contacts ll of. a time relay- I3. The winding of this time relay I3 is con.

nected in series with the contacts H of the switch 3 and contacts 48 of relay 9 so that the relay i3 does not open its contacts i2 until after the motor I has been in operation as an induction motor for a predetermined time.

To prevent the field switch 6 from being closed at the instant the switch 3 is closed to start the motor, the circuit of the closing coil 8 of the field switch 6 also includes the contacts l5 of a time relay IS. The winding of the relay I6 is connected in parallel with the winding of the relay l3 so that the relay l6 closes its contacts l5 and places the closing coil 8 under the control of the speed-responsive relay 3 a predetermined time after the switch 3 is closed. The time setting of the relay l6, however, is relatively short compared with the time setting of the time relay 13. k

The operation of the arrangement shown in the drawing is as follows: When the motor I is at rest and the switch 3 is closed, a current of line frequency is induced in the field winding 4 and this current flows through the discharge resistor l. A portion of this current is rectified by the rectifier Ill and flows through the winding of the relay 9 so that it immediately opens its contacts H. A short time after the switch 3 is closed, the relay l8 closes its contacts IE, but in the meantime, the relay 9 has operated to open its contacts ll so that the circuit of the closing coil 8 is not completed by the closing of the contacts l5. As the speed of the motor increases after the switch 3 is closed, the frequency of the current induced in the lield winding 4 decreases. Therefore, as the speed of the motor increases, the intervals of zero current through the winding of the relay 9 increase in length, but due to the construction of the relay, it does not close its contacts I! until the motor reaches a predetermined speed near synchronous speed, when the interval of zero current through the winding of the relay 9 is of suflicient duration to allow the relay to close its contacts IT. The closing of the contacts I! completes a circuit for the closing coil 8 of the field switchi across the source of excitation 5. The field switch 8 then closes to connect the field winding 4 across the source of excitation 5, and to disconnect the discharge resistor l and the winding of relay 9 from in series relation with field winding 4.

It the load on the motor is of such a value that the motor speed does not increase to a value sufficient to permit the relay 9 to close its contacts I! within the time setting of the relay l3, this relay, by opening its contacts l2, inserts the resistor H in series with the rectifier ID and winding of the relay 9. The insertion of the resistor H in this circuit reduces the current through the winding of the relay 9 sufilciently to cause the relay to close its contacts I! and thereby complete the circuit of the closing coil 8 of the field switch 8. v

In the arrangement shown in the drawing, it will be seen that when the motor starts under heavy loads, the relay 9 will not close its contacts l1 until after the time relay l3 has operated to open its contacts l2. However, on light loads where the synchronizing is easier, the relay will close its contacts I! before the relay 1! opens its contacts.

While I have, in accordance with the patent statutes, shown and described my invention as applied to a particular system and as embodying various devices diagrammatically indicated, changes and modifications will be obvious to those skilled in the art, and I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and.

scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:-

1. In combination, a synchronous motor having an armature winding and a field winding, means for starting said motor including a source of alternating current and switching means for controlling the connection of said source to said armature winding, a source of excitation, means controlled by the speed of said motor normally arranged to eflect the connection of said source excitation to said field winding when the motor speed reaches a predetermined value, and

timing means controlled by said starting means for causing said speed controlled means to eiiect the connection of said source of excitation to said field winding at a motor speed below said firstrnentioned predetermined value but above a second predetermined value.

2. In combination, a synchronous motor having an armature winding and a field winding, means for starting said motor including a source of alternating current and switching means for controlling the connection of said source to said armature winding, a source of excitation, means controlled by the current induced in said field winding normally arranged to effect the connection of said source of excitation to said field winding when the motor speed reaches a predetermined value, and timing means controlled by said startingmeans for causing said induced current controlled means to effect the connection of said source of excitation to said field winding at a motor speed below said first-mentioned predetermined value but above a second predetermined value.

3. In combination, a synchronous motor having an armature winding and a field winding, an alternating current circuit, means for connecting said armature winding to said circuit, a control device, unidirectional conducting means connected in series relation with said field winding, means controlled by the current through said unidirectional conducting means for efi'ecting the operation of said control device, and timing means for decreasing the current flowing through said unidirectional conducting means to efiect the operation of said control device a predetermined time after said motor is started.

4. In combination, a synchronous motor having an armature winding and a field winding, an alternating current circuit, means for connecting said armature winding to said circuit, a source of excitation, switching means for connecting said source of excitation to said field winding, unidirectional conducting means connected in series relation with said field winding, means including a time relay having a winding energized in accordance with the current flowing through said unidirectional conducting means for effecting the operation of said switching means, impedance means, and timing means for connecting said impedance means in circuit with said unidirectional conducting means and said field winding to decrease the current through said relay winding a predetermined time after said motor is started.

5. In combination, a synchronous motor having an armature winding and a field winding, an alternating current circuit, means for connecting said armature winding to said circuit, a source of excitation, switching means for connecting said source of excitation to said field winding, unidirectional conducting means connected in series relation with said field winding, means including a time relay having a winding energized in ac-. cordance with the current flowing through said unidirectional conducting means for efiecting the operation of said switching means, impedance means, and timing means controlled by said connecting means for connecting said impedance means in circuit with said unidirectional conducting means and said 'field winding to effect a de crease in the current through said relay winding alter said armature winding has been energized for a predetermined time.

ARTHUR H. LAUDER.

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